The EPC Class-1 Generation-2 UHF RFID standard is widely accepted UHF standard. This protocol defines interrogator-talks-first RFID system. The interrogator commands are encoded by PIE (pulse interval encoding). Command replies are backscattered at BLF (backscatter-link frequency). The BLF is determined by length of a TRcal symbol which is part of the defined Query command.
The length of the TRcal symbol must be measured precisely to full-fill BLF tolerances given by the EPC standard. The length of the symbol can be measured by using a timer counting number of internal oscillator clock cycles between start and end of the TRcal symbol, the start/end of the symbol is determined from re-sampled demodulator output. It can be used also clock-less methods based for example on capacitor charging such as described in the article of Sung-Jin Kim; Min-Chang Cho; Joonhyun Park; Kisuk Song; Yul Kim; SeongHwan Cho entitled “An ultra-low power UHF RFID tag front-end for EPCglobal Gen2 with novel clock-free decoder” of Circuits and Systems, 2008. ISCAS 2008. IEEE International Symposium on, vol., no., pp. 660, 663, 18-21 May 2008.
The clock-less methods are not widely used because they are not so robust and easy to implement as timer based method.
For timer based method, the precision of length measurement is related to internal oscillator clock frequency—the higher the frequency, the smaller re-sampling error, the better symbol start/end detection, the more precise measurement. On the other hand, the higher the frequency, the higher power consumption. Several studies were done to determine minimum internal oscillator clock frequency which fulfils EPC Class-1 Generation-2 BLF requirements. One can cite for that the article in Impinj Inc., entitled “Gen 2 Tag Clock Rate—What You Need to Know” 2005, or the article of Qiasi Luo; Li Guo; Qing Li; Gang Zhang; Junyu Wang, entitled “A low-power dual-clock strategy for digital circuits of EPC Gen2 RFID tag” RFID, 2009 IEEE International Conference on, vol., no., pp. 7, 14, 27-28 April 2009.
In summary, two options are used. First option, it is used the timer based method, which uses an oscillator with frequency 1.92 MHz or higher (the multiples of highest defined BLF 640 kHz). Second option, it is used an alternative dynamically calibrated methods as defined in the patent application US 2011/0156871 A1, and the patent U.S. Pat. No. 8,193,912 B1.
Again, the alternative dynamically calibrated methods are not so robust and easy to implement as timer based method.
A power optimization of timer based method was proposed in the article of Qiasi Luo; Li Guo; Qing Li; Gang Zhang; Junyu Wang, entitled “A low-power dual-clock strategy for digital circuits of EPC Gen2 RFID tag” RFID, 2009 IEEE International Conference on, vol., no., pp. 7, 14, 27-28 April 2009.